Light source lamp for atomic light absorption analysis

ABSTRACT

In a lamp used as light source for atomic light absorption analysis, comprising a cathode having a hollow therein, an anode disposed in the vicinity thereof, a hermetical envelope to enclose the cathode and the anode, and gaseous atmosphere contained in envelope, the cathode is formed of an alloy composed of silver and at least one metal having a melting point equal to or lower than 500*C and emitting the same spectral line as the metal to be analyzed so that the cathode is prevented from being deformed and the luminous intensity and the analytic accuracy are improved.

United States Patent Hosoya et a1. Aug. 5, 1975 [54] LIGHT SOURCE LAMPFOR ATOMIC 3,422,301 l/l969 Sebens et a1. 313/346 R X LIGHT ABSORPTIONANALYSIS 3,623,136 1 H1971 TOmlta et a1. 313/178 3,725,716 4/1973Yamasaki v. 313/218 X [75] In n Ak r ym Mskoto Tadokoro, 3,820,487 11974 Tomita et a1. 1. 313/311 x both of Hitachi; adami Tomita, Katsuta;Yoji Arai, Katsuta; Shinji Mayama, Katsum, all of Japan PrimaryE.iammerP almer C. Demeo [73] A H} h Ltd J Attorney, Agent, or FzrmCra1g& Antonelll ss1gnee: lac 1, apan [22] Filed: May 9, 1974 57 ABSTRACT[21] Appl. No.: 468,528 1 In a lamp used as light source for atomiclight absorption analysis, comprising a cathode having a hollow [3O]Forelgn Appthcatlon pnonty Data therein, an anode disposed in thevicinity thereof, a May 11.1973 Japan 48-51644 hermetical envelope toenclose the Cathode and the anode, and gaseous atmosphere contained inenve- [52] 313/178 313/2095 lope, the cathode is formed of an alloycomposed of 51 I Cl 61 0 8 silver and at least one metal having amelting point f 346 equal to or lower than 500C and emitting the same 10 I spectral line as the metal to be analyzed so that the cathode isprevented from being deformed and the lu- References Cited minousintensity and the analytic accuracy are improved. UNITED STATES PATENTSI 3,089,054 5/1963 Walsh ct al. 313/209 X 10 Claims, 15 Drawing FiguresSi-TEE? PATENTEU AUG 5|975 SHEET 2 EMmEZEW I MQW FIG. 7

PATENIEUIUB 51% 3,898,501

F|G.8 I50 IIIII LUMINOUS INTENSITY (RELATIVE VALUE) l I I I II ISTABILITY TIME (MINUTE) 0 IO 203040506070809OIU) CONTENT OF CADMIUM INALLOY (ATOMIC PAIENTED 5I975 3.898.501

SHEET 7 FIG. 9

IIIIII STABILITY TIME (MINUTE) LUMINOUS INTENSITY (RELATIVE VALUE) l O m4 m 00 6 K; 3 0 5 8 lb o3'o4 05 06bToao9omo CONTENT OF TIN IN ALLOY(ATOMIC pmgmmus 51915 3,898,501

SHEET 8 FIG. I0

I lllIl I x -IO LUMINOUS INTENSITY (RELATIVE VALUE) 5 3 STABILITY TIME(MINUTE) I. lllllllllj CONTENT OF LEAD IN ALLOY (ATOMIC%I PATENTEU AUG 5I975 FIG.

T 1 N E On R U C n w 2 E M 0/0 l A o o w n u 0/ O/ 1 0 2 m v 0 O 2PATENTED 5M5 3,898,501

' SHEET 10 5 FIG. l2

0 Zn i E a z 0 E g (I) 0 8m; Z g

22 I llllllllo l lllllliz DISCHARGE CURRENT (mA) SHEET FIG.

I llllllll I02 DISCHARGE CURRENT (mA) mi H mo o P O l H ///.I I w W mLIGHT SOURCE LAMP FOR ATOMIC LIGHT ABSORPTION ANALYSIS The presentinvention relates to a lamp used as light source for atomic lightabsorption analysis, and more particularly to the constitution of thecathode of such a lamp.

The atomic light absorption analysis based on the principle of atomiclight absorption is used for the quantitative analysis of the metalsalts contained in a sample solution.

The atomic light absorption analysis is adapted to the case where thequantity of the sample is very small and very frequently used in thefield of medicine, industrial chemistry, food chemistry or oilchemistry. In a typical example of this analysis, a metal salt in asample solution is decomposed into atomic vapor of metal, by means ofthermal energy due to flame and a resonance spectral line from anexternal light source is passed through the metal vapors to be absorbedin the vapors and the degree of absorption of the spectral line givesthe result of analysis. The basic matters concerning the analysis aredisclosed in the US. Pat. No. 2,847,899 specification.

In the method of atomic light absorption analysis, only the spectralline having specific wavelength is absorbed in the atomic vapor and theanalysis is effected on the basis of the predetermined relationshipexisting between the quantity of absorbed light and the concentration ofthe atom to be analyzed. Accordingly, even if other kinds of atomscoexist in the material forming the cathode of the light source lampused in the analysis, there is no nuisance unless there is any overlapof spectral lines. Therefore, the cathode may ,be formed of any suitablealloy or metal composite.

In practice, the cathode is formed of an alloy of several metals fused,or to prepare a material for use as cathode the powder of metals oralloys are mingled, pressed into shape and sintered or a porous sinteredmaterial is impregnated with a fused metal or alloy having a meltingpoint lower than that of the porous sintered material.

The alloy or composite cathode is widely used in an atomic lightabsorption analyzer adapted especially for the analysis of metal havinga melting point lower than 500C. This is because of the restrictionsimposed upon the fabrication of a lamp used as light source for theanalyzer. Namely, such a light source lamp is fabricated as follows.Electrodes are arranged in place in a glass envelope, inert gas such asargon or neon gas is introduced within the envelope and the open end ofthe envelope is hermetically sealed. At the time of sealing theenvelope, the electrodes are heated up to near 500C. Accordingly, in thecase of analyzing a metal having a melting point lower than 500C, if thecathode is made of the same metal as that to be analyzed so as to emit aresonance spectral line of the metal to be analyzed, the cathode isdeformed due to the heating for sealing the glass envelope so that theproper function of the cathode is damaged. For this reason, the cathodeis formed of alloy or composite metal to have a higher melting point andtherefore a higher resistance to thermal deformation.

In the case of a cathode formed of alloy or composite metal (this termmeans throughout this specification such a sintered body of shapedmetal-powder or a sintered porous body impregnated with fused metal oralloy as mentioned above), the metal (hereafter referred to as the othermetal) combined with a metal which emits the same resonance spectrallines as the metal to be analyzed, needs to have the followingproperties.

I. Where formed of alloy,

la the other metal must form an alloy having a melting point higher than500C,

lb the metal must be able to be shaped into a cathode, and

1c the other metal must not emit light having wavelengths equal to orvery near those of light emitted from the metal to be analyzed.

2. Where formed of composite metal,

2a the composite cathode must have a melting point higher than 500C,

2b the powders are easy to be pressed in the shape of cathode, and

2c the wavelengths must not overlap those of the metal to be analyzed.

In such aspects as mentioned above, many investigations have been madeand it has now been proved preferable to use copper as the other metal.In the case of copper used as the other metal, however, there is a firstdrawback that the kinds of metals (to be analyzed) are strictly limited.The concrete description will be given below. The wavelengthscorresponding to the spectral lines emitted from metals having meltingpoints lower than 500C, are within a range of 2,000 A to 3,000 A. Table1 given below shows the wavelengths of the spectral lines of the metalshaving melting points lower than 500C and it is seen that all the metalsexcept thallium satisfy the condition mentioned above.

On the other hand, copper has wavelengths of spectral lines 2,136 A nearthat of zinc, 2,293 A near that of cadmium, and 3,072 A near that ofbismuth. With a combination of copper with zinc, cadmium and/or bismuth,therefore, some of the spectral lines partially overlap with one anotherso that high accuracy in analysis becomes impossible.

A second drawback of copper used as the other metal is that the luminousintensity is diminished since the concentration quenching phenomenon iscaused owing to the self-absorption due to too much vapor of luminousmetal being generated under saturated condition because of the smallsputtering rate of copper. The phenomenon takes place as follows. Atsputtering, copper and metal having a low melting point, in the form ofsmall particles or atoms are emitted from the surface of the cathode.Since copper has a small sputtering rate, the metal having a low meltingpoint occupies the major part of the sputtered particles. Accordingly,unexcited atoms or particles exit in excess of electrons to be collidedwith during excitation so that the selfabsorption of light, i.e.,concentration quenching phenomenon, takes place to diminish the luminousintensity. The luminous intensity can be increased to a certain extentby increasing the discharge current, but the increase in the dischargecurrent will be accompanied by the degradation in the life time of thecathode. Moreover, the analysis is started after the sputtered particleshave reached saturation and the spectral lines have been stabilized. Andwhen such an element like copper as having a small sputtering rate isincluded, the time when the sputtered atoms or particles reach apredetermined concentration is so slowly reached that the warming-upperiod, i.e., the period during which the emitted spectral linesconverge into stability, is too long.

It is, therefore, one object of the present invention to provide acathode which has a high accuracy in analysis even if the other metal iscombined with each or some of many kinds of metals having melting pointslower than 500C.

Another object of the present invention is to provide a cathode in whichthe other metal has so large a sputtering rate that the self-absorptionof light may hardly take place.

An additional object of the present invention is to provide a cathodewhich has a greater luminous intensity and a shorter warming-up periodthan a conventional cathode using copper as the other metal.

According to the present invention, which has been made to attain theabove objects, the cathode is formed of an alloy of silver and at leastone of the metals having melting points lower than 500C and emittingresonance spectral lines. Silver does not emit light having a wavelengthlying between 2,000 A and 3,000 A, nor has a spectral line overlappingthe spectral line of thallium, i.e., a wavelength of 4,096 A. Therefore,silver can be combined with any of the metals given in the Table l andalso can attain a high accuracy in analysis. Moreover, according to thepresent invention, the hollowed cathode must be made of an alloyprepared through melting. Other methods besides alloying through meltingcan not attain satisfactory luminous intensity and accuracy in analysis.In the case where the cathode is not formed of an alloy made throughmelting but of composite metal, the powder metallurgy techniques must beusually used. The composite metal prepared through the powder metallurgycontains a considerable amount of gas, especially oxygen and the oxygencan not be completely removed even through degassing operation in thelamp fabrication procedure, so that the lamp quality is very muchdegraded, that is, the luminous intensity is decreased and the accuracyin analysis is also degraded. On the other hand, through alloyingtechniques, the melting of metals in vacuum or non-oxidizing atmospherecan shut out oxygen so that the drawback inevitable with the powdermetallurgy can be eliminated. Thus, high luminous intensity and highaccuracy in analysis can be attained.

Silver has a much larger sputtering rate than copper. Therefore, by theuse of silver, the concentration of the sputtered atoms of the metalunder its unexcited state can be kept small, which metal has a lowmelting point and emits the resonance spectral lines. Accordingly, theself-absorption of light can be avoided so that the concentrationquenching phenomenon never takes place. Moreover, the time during whichthe sputtered particles reach saturation in the lamp envelope is shorteras compared with the case where copper is used in place of silver andthe time of starting analysis can be quickly reached.

The cathode provided according to the present invention can also beapplied to the analysis of plural metals having melting points lowerthan 500C. For this purpose, the cathode is formed of a multi-elementalloy of silver and plural metals which can emit the same spectral linesas the metals to be analyzed. In the formation of such a multi-elementalloy, however, it is necessary to except a combination of zinc andtellurium whose spectral lines are proximate to each other anda-combination of zinc and lead which are separated from each other andcan not form an alloy. An alloy of cadmium, zinc and silver or an alloyof cadmium, lead and silver is suitable for a cathode used in aso-called multi-element lamp employed in the analysis of pluralelements.

The preferable composition of a zinc-silver alloy cathode is 20 to 80at. percent (atomic percent), especially 40 to 60 at. percent of Zincand silver as the rest. If zinc is less than 20 at. percent, theluminous intensity becomes very poor, and if zinc is more than 80 at.percent, the resultant alloy becomes brittle so that the cutting of thealloy into shape is almost impossible. The preferable composition of abismuth-silver alloy cathode is 40 to 80 at. percent of bismuth andsilver as the rest. If bismuth is less than 40 at. percent, the luminousintensity is poor, and if it is more than 80 at. percent, the periodduring which the spectral lines are stabilized, i.e., the warming-upperiod, rapidly increases. Especially, 50 to percent of bismuth gives anoptimum condition.

The cadmium-silver alloy cathode has a preferable composition of cadmium15 to 70 at. percent and silver as the rest and the luminous intensityof the cathode takes the maximum value when cadmium is 50 at. percentand decreases if the atomic percentage of cadmium is less or more than50 at. percent. And the warming-up period increases with the increase inthe content of cadmium.

The tin-silver alloy cathode has a preferable composition of time 30 to70 at. percent and silver as the rest.

The lead-silver alloy cathode has a preferable composition of lead 30 toat. percent, especially 50 to 70 at. percent, and silver as the rest.

The selenium-silver alloy cathode has a preferable composition ofselenium 33 to 40 at. percent and silver as the rest.

The cadmium-lead-silver alloy cathode has an optimum composition ofcadmium 15 to 30 at. percent, lead 40 to 60 at. percent and silver asthe rest. If the amount of cadmium is excessive or the amount of lead isdecreased, then the sputtered particles of cadmium abnormally increaseso that the concentration quenching phenomenon takes place.

In the case of the cadmium-zinc-silver alloy cathode,

the concentration quenching phenomenon tends to be caused since thevapor pressures of cadmium and zinc are both high, so that the amount ofsilver in the alloy should be increased to control the relativeconcentrations of Cd, Zn and Ag. A preferable composition consists ofcadmium 5 to 30 at. percent, zinc l0 to 30 at. percent and silver as therest.

Each of the alloys given above may be prepared through melting in inertgas if each constituent is sufficiently deoxidized and has a purity ofhigher than 99.9

percent. If, however, each constituent is not sufficiently deoxidized,the melting of the constituents should be performed in vacuum and theforced deoxidization of the constituents should be carried out in vacuumfor a sufficient time so that the purity of the resultant alloy may behigher than 99.9 percent. The alloy in the molten state is then pouredinto a mold having the shape of cathode to be used and finally finishedafter necessary cutting. The completed cathode is placed in a glassenvelope, the glass envelope is degassed, inert gas such as argon, neonor helium gas is introduced within the envelope, and the end of theenvelope is hermetically sealed. The degassing operation is usuallyperformed to such an extent that the pressure in the envelope is about 5X mmHg. Neon gas at pressures of 5 to 9 mmHg is usually contained in theenvelope.

Other features and advantages of the present invention will be apparentwhen one reads the following description of the preferred embodimentswith the aid of the attached drawings in which:

FIG. 1 is a longitudinal cross section of a lamp used as light source inan atomic light absorption analyzer;

FIG. 2 shows the structure of the electrodes of a lamp as shown in FIG.1;

FIG. 3 shows in graphical representation two curves, one indicating theluminous intensity of the zinc spectral line emitted from a light sourcelamp using a zincsilver alloy cathode and the other representing thetime required for the zinc spectral line to be stabilized;

FIG. 4 shows the spectrum of light emitted from silver;

FIG. 5 shows the spectrum of light emitted from copper;

FIG. 6 shows in graphical representation two curves, one indicating theluminous intensity of the bismuth spectral line emitted from a lightsource lamp using a bismuth-silver alloy cathode and the otherrepresenting the time required for the zinc spectral line to bestabilized;

FIG. 7 shows in graphical representation two curves, one indicating theluminous intensity of the selenium spectral line emitted from a lightsource lamp using a selenium-silver alloy cathode and the otherrepresenting the time required for the selenium spectral line to bestabilized;

FIG. 8 shows in graphical representation two curves, one indicating theluminous intensity of the cadmium spectral line emitted from a lightsource lamp using a cadmium-silver alloy cathode and the otherrepresenting the time required for the cadmium spectral line to bestabilized;

FIG. 9 shows in graphical representation two curves, one indicating theluminous intensity of the time spectral line emitted from a light sourcelamp using a tinsilver alloy cathode and the other representing the timerequired for the tin spectral line to be stabilized;

FIG. 10 shows in graphical representation two curves, one indicating theluminous intensity of the lead spectral line emitted from a light sourcelamp using a lead-silver alloy cathode and the other representing thetime required for the lead spectral line to be stabilized;

FIGS. 11 and 12 show in graphical representation the relationshipsbetween the luminous intensity of a light source lamp using acadmium-zinc-silver alloy cathode and the discharge current; and

FIGS. 13 to 15 show in graphical representation the relationshipsbetween the luminous intensity of a light source lamp using acadmium-lead-silver alloy cathode and the discharge current.

A lamp used as light source in an atomic light absorption analyzerusually has such a structure as shown in FIG. 1. In FIG. 1, a cathode lis formed of an alloy of silver and other elements which emit the sameresonance spectral lines as the metals to be analyzed. The cathode 1 hasa cylindrical form with a hollow 2 therein to increase the luminousintensity. The resonance spectral lines are created in the hollow 2. Ananode 3 has a ring-like form and the discharge between the cathode 1 andthe anode 3 gives rise to the resonance spectral lines. The cathode 1and the anode 3 are connected respectively with a cathode lead 4 and ananode lead 5. A discharge protection plate 6 is made of, for example,mica. An insulating tube 7 is usually made of steatite. An insulatingtube 8 is provided for the anode lead 5. A metal tube 9 made of, forexample, nickel covers the surface of the cathode 1 except that of thehollow 2 to create the resonance spectral lines therein. The metal tube9 serves to prevent the discharge between the anode 3 and the surface ofthe cathode 1 except that of the hollow 2. A hermetical envelope 10 isusually made of transparent glass. The hermetic envelope 10 containstherein inert gas such as argon, neon or helium gas. The envelope 10 iscoupled to a base socket 11 by means ofa metal tube 12 made of, forexample, nickel. A window 13 through which the resonance spectral linesare emitted is usually made of quartz glass.

When an appropriate voltage is applied between the cathode 1 and theanode 3 of the lamp having the structure described above, the dischargecurrent flows between the electrodes 1 and 3. Accordingly, the inert gasin the envelope 10 is ionized to produce positive ions, which bombardthe surface of the hollow 2. As a result, the atoms of the metalsforming the cathode 1 are evaporated due to sputtering effect and Joulesheating, and the vaporized atoms are excited in the hollow 2 to emit theresonance spectral lines.

FIG. 2 shows the rough structure of the electrodes of the lamp shown inFIG. 1. In FIG. 2, the arrows indicate the direction of travel of lighthaving the resonance spectral lines. The discharge between the cathode land the anode 3 is controlled to occur in the normal glow region inorder to suppress the consumption of the cathode body throughsputtering, and especially in that normal glow region which lies nearthe abnormal glow region.

EMBODIMENT 1 Silver having a purity of 99.9 percent and zinc having apurity of 99.9 percent were fused in argon gas and nine kinds of alloyshaving different compositions were formed. The alloys in the moltenstate were poured into molds and cast into cathodes. The cathodes werefinished after necessary cutting. Each of the finished cathodes has anouter diameter 8 mm, a height 20 mm, an inner diameter of cylindricalhollow 4 mm and a depth of the hollow 15 mm. Each of the cathodes wasplaced together with the mated anode and other parts in a glass envelopeto fabricate a lamp as shown in FIG. 1. Neon gas at pressure of 9 mmHgwas contained in the envelope. With the lamps equipped with thecathodes, the luminous intensity of the zinc spectral line and the time(hereafter referred to as stability time for brevity) during which thespectral line is stabilized were'measured. The discharge current was mA.The result of the measurement is shown in FIG. 3. The luminous intensityis plotted in the relative values to a lamp equipped with a cathodeformed of an alloy consisting of zinc 10 at. percent and silver as therest, the luminous intensity of which is assumed to be 10. The luminousintensity is represented in solid curve 21 while the stability time isrepresented in dashed curve 20. The luminous intensity is maximum whenthe content of zinc in the alloy is 50 at. percent and it is loweredwhen the content is more or less than 50 at. percent. The stability timetends to be shorter with the increase in the content of silver in thealloy. With a cathode of pure zinc, the relative value of the luminousintensity is 20 and the luminous intensity of the light source lamp canbe increased by using a cathode formed of zinc-silver alloy containingzinc of more than at. percent.

As a conventional cathode used in a light source lamp for the analysisof zinc is known a sintered body consisting of zinc and copper powdersmixed in a ratio of l l. The luminous intensity of this sintered cathodeis about 30 if the intensity of the cathode of an alloy consisting ofzinc 10 at. percent and silver as the rest is assumed to be 10. And thestability time of the sintered cathode is about 30 minutes. The cathodeof zincsilver alloy has a stability time by far shorter than that of thesintered cathode and a cathode of an alloy consisting of zinc to 80 at.percent and silver as the rest has a high luminous intensity, too.

Thus, a zinc-silver alloy cathode has a higher luminous intensity and ashorter stability time than zinccopper sintered cathode. This is due tothe fact that the sputtering rate of silver is greater than that ofcopper and therefore the concentration quenching phenomenon hardly takesplace.

FIGS. 4 and 5 respectively show the spectral lines of silver and copper.It is seen from the inspection of FIGS. 4 and 5 that silver has nospectral line in the vicinity of the spectral line at 2,138.6 A of zincwhile copper has a spectral line at 2,136 A. Therefore, with azinc-copper sintered cathode, the spectral lines of zinc and copperoverlap each other so that the accuracy in analysis tends to be lowered.

EMBODIMENT 2 Bismuth-silver alloy cathodes were formed the dimensionsand the fabricating method of which were the same as the cathode in theembodiment 1. FIG. 6 shows the luminous intensity and the stability timeas the result of measurement with discharge current of 10 mA. Theluminous intensity is represented in relative values to a cathode ofalloy consisting of bismuth 10 at. percent and silver as the rest, theluminous intensity of which is assumed to be 10. The luminous intensityis especially large when the content of zinc in the alloy is 50 to 80at. percent. The stability time is very short, that is, shorter than 14minutes, when the content of silver in the alloy is more than 30 at.percent.

EMBODIMENT 3 Light source lamps equipped with selenium-silver alloycathodes were fabricated according to the same process as in theembodiment l and the luminous intensity and the stability time of theselenium spectral line were measured with the discharge current of 10mA. The result of the measurement is shown in FIG. 7. The

ity.

EMBODIMENT 4 Cathodes of cadmiumsilver alloys and a cathode of cadmiumonly were formed according to the same proce'ss as in the embodiment 1and the luminous intensity and the stability time of the cadmiumspectral line were measured with discharge current of 10 mA. The resultof the measurement is shown in FIG. 8. The luminous intensity isrepresented in relative values to a cathode of alloy composed of cadmium10 at. percent and silver as the rest, the luminous intensity of whichis assumed to be 10. The cadmium-silver alloy cathode gives the maximumluminous intensity when the content of zinc in the alloy is 50 at.percent. The stability time is shorter with the increase in the contentof silver in the alloy and especially short for silver content of morethan 30 at. percent.

EMBODIMENT 5 Lamps having tin-silver alloy cathodes and a lamp having acathode of tin only were fabricated according to the same process asused in the embodiment I and the luminous intensity and the stabilitytime of the tin spectral line were measured with discharge current of 10mA. FIG. 9 shows the result of the measurement. The luminous intensityis represented in relative values to a cathode of alloy composed of tin10 at. percent and silver as the rest, the luminous intensity of whichis assumed to be 10. The luminous intensity is very high when thecontent of tin in the alloy is 50 to at. percent.

EMBODIMENT 6 Lamps having lead-silver alloy cathodes and a lamp having acathode of lead only were fabricated according to the same process as inthe embodiment l and the luminous intensity and the stability time weremeasured with the discharge current of 10 mA. FIG. 10 shows the resultof the measurement. The luminous intensity is represented in relativevalues to a cathode of alloy composed of lead 10 at. percent and silveras the rest, the luminous intensity of which is assumed to be 10. Thelead-silver alloy cathode gives the maximum luminous intensity when thecontent of lead in the alloy is 60 at. percent and the stability time isvery short when the content of silver in the alloy is more than 20 at.percent. Since the stability time of a conventional cathode which wasfabricated by impregnating a porous body of sintered copper with leadand has the composition of copper at. percent and lead 25 at. percent,is about 40 minutes, then the cathode according to the present inventioncan be claimed to have an excellent quality.

EMBODIMENT 7 Lamps having cathodes of alloys composed respectively ofcadmium 10 at. percent, zinc 10 at. percent and silver as the rest andof cadmium I0 at. percent,

zinc l5 at. percent and silver as the rest, were so fabricated as tohave such a structure as shown in FIG. 1. The dimensions and thefabricating process of each cathode were the same as those of thecathode in the embodiment l and the luminous intensities of the cathodeswith respect to the zinc spectral line at a wavelength 2138.6 A and thecadmium spectral line at a wavelength 2,288 A were measured. FIG. 11shows the plotting of the luminous intensities of the cadmium and zincspectral lines of a cathode of alloy composed of cadmium 10 at. percent,zinc 10 at. percent and silver as the rest, against the dischargecurrent. The luminous intensities are represented in relative valueswhich was defined by assuming the intensity of the cadmium spectral lineto be 50 when the discharge current is 8 mA. FIG. 12 shows the plottingof the luminous intensities of the cadmium and zinc spectral lines of acathode of alloy composed of cadmium 10 at. percent, zinc 15 at. percentand silver as the rest, against the discharge current. In this case, theluminous intensities are represented in relative values defined byassuming the intensity of the cadmium spectral line to be 70 when thedischarge current is 10 mA. It is seen from FIGS. 11 and 12 that ahigher luminous intensity can be obtained for the same discharge currentin FIG. 12 than in FIG. 11. The stability times of the cadmium and zincspectral lines of the cathode of alloy composed of cadmium 10 at.percent, zinc 10 at. percent and silver as the rest are about l minutesand about 6 minute, respectively and those of the cadmium and zincspectral lines of the cathode of alloy composed of cadmium at. percent,zinc 15 at. percent and silver as the rest are about 17 minutes andabout 10 minutes, respectively.

EMBODIMENT 8 Lamps having cathodes of cadmium-lead-silver alloys were sofabricated as to have such a structure as shown in FIG. 1. Thedimensions and the fabricating process of each cathode were the same asthose of the cathode in the embodiment l and the luminous intensities ofthe cadmium and lead spectral lines were measured. FIGS. l3, l4 and 15respectively show the plottings of the luminous intensities of thecadmium and lead spectral lines of the cathodes of alloys composed ofcadmium 30 at. percent, lead 50 at. percent and silver as the rest, ofcadmium 20 at. percent, lead 60 at. percent and silver as the rest, andof cadmium 20 at. percent, lead 40 at. percent and silver as the rest,against the discharge currents. The luminous intensity of the leadspectral line varies almost in the same rate independent of thecomposition of the alloy, as seen from FIGS. l3, l4 and 15, while thatof the cadmium spectral line varies in different manners depending uponthe composition of the alloy. The cathode of alloy composed of cadmium20 at. percent, lead 40 at. percent and silver as the rest, gives themaximum intensity of cadmium spectral line for a given dischargecurrent. The stability times of the cadmium and lead spectral lines ofthe cathode of alloy composed of cadmium 20 at. percent, lead 60 at.percent and silver as the rest, were about 10 minutes and about l5minutes, respectively.

What we claim is:

1. A lamp used as light source for atomic light absorption analysis,comprising a cathode having a hollow, an anode arranged in the vicinityof said cathode, a hermetical envelope to enclose said cathode andanode, and gaseous atmosphere contained in said envelope, wherein saidcathode is formed of a molten alloy of silver and at least one metalselected from the group consisting of zinc, bismuth, cadmium, tin andlead and which can emit the same resonance spectral line as the metal tobe analyzed, and said alloy has a purity equal to or higher than 99.9percent.

2. A lamp used as light source for atomic light absorption analysis, asclaimed in claim 1, wherein said cathode is formed of an alloy composedof zinc 20 to 80 at. percent and silver as the rest.

3. A lamp used as light source for atomic light absorption analysis, asclaimed in claim 1, wherein said cathode is formed of an alloy composedof bismuth 40 to 80 at. percent and silver as the rest.

4. A lamp used as light source for atomic light absorption analysis, asclaimed in claim 1, wherein said cathode is formed of an alloy composedof cadmium 15 to at. percent and silver as the rest.

5. A lamp used as light source for atomic light absorption analysis, asclaimed in claim 1, wherein said cathode is formed of an alloy composedof tin 30 to 70 at. percent and silver as the rest.

6. A lamp used as light source for atomic light absorption analysis, asclaimed in claim 1, wherein said cathode is formed of an alloy composedof lead 30 to at. percent and silver as the rest.

7. A lamp used as light source for atomic light absorption analysis, asclaimed in claim 1, wherein said cathode is formed of an alloy composedof cadmium 15 to 30 at. percent, lead 40 to 60 at. percent and silver asthe rest.

8. A lamp used as light source for atomic light absorption analysis, asclaimed in claim 1, wherein said cathode is formed of an alloy composedof cadmium 5 to 30 at. percent, zinc 10 to 30 at. percent and silver asthe rest.

9. A lamp used as light source for atomic light absorption analysis, asclaimed in claim 1, wherein said cathode is formed of an alloy composedof silver having a purity equal to or higher than 99.9 percent and metalhaving a purity equal to or higher than 99.9 percent.

10. A lamp used as light source for atomic light absorption analysis, asclaimed in claim 1, wherein each of said at least one metal has amelting point lower than 500C.

1. A LAMP USED AS LIGHT SOURCE FOR ATOMIC LIGHT ABSORPTION ANALYSISCOMPRISING A CATHODE HAVING A HOLLOW, AN ANODE ARRANGED IN THE VICINITYOF SAID CATHODE, A HERMETICAL ENVELOPE TO ENCLOSE SAID CATHOE AND ANODE,AAND GASEOUS ATMOSPHERE CONTAINED IN SAID ENVELOPE, WHEREIN SAID CATHODEIS FORMED OF A MOLTEN ALLOY OF SILVER AND AT LEAST ONE METAL SELECTEDFROM THE GROUP CONSISTING OF ZINC, BISMUTH, CADMIUM, TIN AND LEAD ANDWHICH CAN EMIT THE SAME RESONANCE SPECTRAL LINE AS THE METAL TO BEANALYZED AND SAID ALLOY HAS A PURITY EQUAL TO OR HIGHER THAN 99.9PERCENT.
 2. A lamp used as light source for atomic light absorptionanalysis, as claimed in claim 1, wherein said cathode is formed of analloy composed of zinc 20 to 80 at. percent and silver as the rest.
 3. Alamp used as light source for atomic light absorption analysis, asclaimed in claim 1, wherein said cathode is formed of an alloy composedof bismuth 40 to 80 at. percent and silver as the rest.
 4. A lamp usedas light source for atomic light absorption analysis, as claimed inclaim 1, wherein said cathode is formed of an alloy composed of cadmium15 to 70 at. percent and silver as the rest.
 5. A lamp used as lightsource for atomic light absorption analysis, as claimed in claim 1,wherein said cathode is formed of an alloy composed of tin 30 to 70 at.percent and silver as the rest.
 6. A lamp used as light source foratomic light absorption analysis, as claimed in claim 1, wherein saidcathode is formed of an alloy composed of lead 30 to 80 at. percent andsilver as the rest.
 7. A lamp used as light source for atomic lightabsorption analysis, as claimed in claim 1, wherein said cathode isformed of an alloy composed of cadmium 15 to 30 at. percent, lead 40 to60 at. percent and silver as the rest.
 8. A lamp used as light sourcefor atomic light absorption analysis, as claimed in claim 1, whereinsaid cathode is formed of an alloy composed of cadmium 5 to 30 at.percent, zinc 10 to 30 at. percent and silver as the rest.
 9. A lampused as light source for atomic light absorption analysis, as claimed inclaim 1, wherein said cathode is formed of an alloy composed of silverhaving a purity equal to or higher than 99.9 percent and metal having apurity equal to or higher than 99.9 percent.
 10. A lamp used as lightsource for atomic light absorption analysis, as claimed in claim 1,wherein each of said at least one metal has a melting point lower than500*C.